Makefile: fix parallel building

This changes the default rules for ar archives to work correctly under
parallel buildings in GNU make ≥4.4.1.

Incidentally, this also fixes the entire build being sequential for
GNU make <4.4 due to .NOTPARALLEL not supporting prerequisites.
For older versions, timestamps of archive members are not handled
correctly, but that's a relatively minor issue.

Makefile

@@ -203,11 +203,7 @@ $(LIBDIR)/libgray.so: $(MODULES) | $(LIBDIR)
 	$(LD) -shared $(LDFLAGS) -o '$@' $^
 
 # GRAY static library
-$(LIBDIR)/libgray.a($(MODULES)): | $(LIBDIR)
-$(LIBDIR)/libgray.a: $(LIBDIR)/libgray.a($(MODULES))
-
-# Don't update archives in parallel, it's unsupported
-.NOTPARALLEL: $(LIBDIR)/libgray.a
+$(LIBDIR)/libgray.a: $(LIBDIR)/libgray.a($(MODULES)) | $(LIBDIR)
 
 # GRAY static library when compiling from JETTO
 -include ../include.mk
@@ -270,6 +266,12 @@ $(OBJDIR)/%.d: %.f90 | $(OBJDIR)
 	 sort -u >> '$@'
 	@sed -nE 's@^#include "(.+)"@$(INCDIR)/\1 \\@p' '$<' >> '$@'
 
+# Accumulate members and update archives all at once
+# Note: this replaces the default POSIX rules that
+# break under parallel building (make -j)
+(%) : % ;
+%.a : ; $(AR) $(ARFLAGS) $@ $?
+
 # Load the generated rules
 include $(DEPS)